An accurate knowledge of the hemodynamic status/cardiac output of the heart of a patient helps medical practitioners assess a patient's medical condition. The constituents of cardiac output (measured, for example, in liters/minute), heart rate (measured, for example in beats per minute) and stroke volume (measured for example in mls) may also provide useful information. The stroke volume, or cardiac stroke volume, is the volume of blood ejected by the left ventricle during systole across the aortic valve forwards into the aorta during each cardiac contraction. This volume normally corresponds to the volume of blood in the left ventricle at the end of the systole minus the pre-systole diastolic volume of the left ventricle. This is particularly true in acute situations, such as, for example, for patients in intensive care units or patients undergoing an operation where for example it is used in fluid and drug management during anaesthesia and after. Knowledge of a patient's cardiac output, or its constituents, may, moreover, be beneficial in less critical or less life threatening situations, such as in situations where the monitoring of the patient is generally desirable.
It has long been known that cardiac output as well a stroke volume and heart rate can be determined based on an analysis of arterial blood pressure waveform. Summaries of previously suggested methods for the determination of the output of the heart are provided in WO 97/00017, a prior patent disclosure of the present assignee, and in A. Rhodes, R. Sunderland, “Arterial Pulse Power Analysis: The LiDCO™ plus System”, Functional Hemodynamic Monitoring Update in Intensive Care and Emergency Medicine, 42, pp. 183-192, 2005, the entireties of which are incorporated herein by this reference.
WO 97/00017 discloses evaluating an arterial pressure waveform by performing a volume transformation followed by an autocorrelation operation applied between a part of the transformed pressure waveform comprising first and second pressure pulses of a patient's heart. An analysis of this nature requires that data representing a second heart beat be first acquired before the data can be analysed and an indication of the patient's hemodynamic status, including stroke volume, heart rate and/or cardiac output, can be provided. The indication of the patient's hemodynamic status can thus only be provided with a slight delay.
It was realised that in acute scenarios it may be desirable for an indication of cardiac output to track the patient's actual cardiac output in real time so that an indication of cardiac output/hemodynamic status can be provided for each heart beat. Such beat-by-beat tracking requires that stroke volume be determined based on pressure data relating to a single heart beat only. It is therefore desirable for data relating to a single heart beat to be extracted from the arterial pressure waveform.